I. Field of the Invention
This invention relates generally to implantable monitoring devices, and more particularly to a method for monitoring the hemodynamic status of a patient with a chronic disease such as heart failure, ischemic heart disease, or diabetes.
II. Description of the Related Art
Many chronic diseases, such as diabetes and heart failure, require close medical management to reduce morbidity and mortality. Because the disease status evolves with time, frequent physician follow-up examinations are often necessary. At follow-up, the physician may make adjustments to the drug regimen in order to optimize therapy. This conventional approach of periodic follow-up is unsatisfactory for some diseases, such as heart failure, in which acute, life-threatening exacerbations can develop between physician follow-up examinations. It is well know among clinicians that if a developing exacerbation is recognized early, it can be easily and inexpensively terminated, typically with a modest increase in oral diuretic. However, if it develops beyond the initial phase, an acute heart failure exacerbation becomes difficult to control and terminate. Hospitalization in an intensive care unit is often required. It is during an acute exacerbation of heart failure that many patients succumb to the disease.
It is often difficult for patients to subjectively recognize a developing exacerbation, despite the presence of numerous physical signs that would allow a physician to readily detect it. This problem is well illustrated by G. Guyatt in his article entitled “A 75-Year-Old Man with Congestive Heart Failure,” 1999, JAMA 281(24)2321–2328. Furthermore, since exacerbations typically develop over hours to days, even frequently scheduled routine follow-up with a physician cannot effectively detect most developing exacerbations. It is therefore desirable to have a system that allows the routine, frequent monitoring of patients so that an exacerbation can be recognized early in its course. With the patient and/or physician thus notified by the monitoring system of the need for medical intervention, a developing exacerbation can easily and inexpensively be terminated early in its course.
The multiplicity of feedback mechanisms that influence cardiac performance places the heart at the center of a complex control network. The neurohumoral axis includes the autonomic nervous system, consisting of sympathetic and parasympathetic branches, and numerous circulating hormones such as catacholamines, angiotensin, and aldosterone. Neural reflex arcs originating from pressure and stretch receptors, which directly measure mechanical hemodynamic status, modulate the neurohumoral axis. Similarly, chemoreceptors respond to changes in CO2, pH, and O2, which assesses cardiopulmonary function. The neurohumoral system influences cardiac performance at the level of the cardiac electrical system by regulating heart rate and the conduction velocity of electrical depolarizations. It also influences cardiac performance at the mechanical level, by controlling contractility, that is, the effective vigor with which the heart muscle contracts. Conventional cardiac monitors, such as defibrillators, pacemakers, Holter monitors, and cardiac event records, are tailored for the diagnosis and/or therapy of abnormalities of the cardiac electrical system. In contrast, heart failure is a disease of the cardiac mechanical system: it is primarily a failure of the myocardium to meet the mechanical pumping demands required of it. In monitoring the status of a heart failure patient, measuring the mechanical hemodynamic variables is clearly desirable. Examples of mechanical hemodynamic variables include atrial, ventricular, and arterial pressures, and cardiac output (volume of blood pumped into the aorta per unit time). However, because of the complex feedback network that monitors and controls cardiac performance, measuring variables that do not directly reflect the mechanical performance of the heart is also useful. In this way, measuring cardiac electrical activity to assess heart rate variability (described below) allows one to infer the state of the autonomic nervous system, which allows one to infer information about the hemodynamic status of a heart failure patient. Similarly, recognition of Cheyne-Stokes respiration (described below) via respiratory pattern analysis, hemoglobin saturation analysis, and blood gas analysis allows one to detect the presence of pulmonary edema, and thereby detect an acute heart failure exacerbation, though none of these parameters directly measure mechanical hemodynamic status.
One approach to frequent monitoring of heart failure patients that has been proposed is the daily acquisition of the patient's weight and responses to questions about subjective condition (Alere DayLink Monitor, Alere Medical, Inc., San Francisco, Calif.). The simplicity and noninvasive embodiment of this approach are desirable features. However, both the amount and the sophistication of the objective physiological data that can be acquired in this way are quite limited, which consequently limits the accuracy of the system. Furthermore, the system requires the active participation of the patient, who must not deviate from the precise data acquisition routine or risk introducing confounding factors into the acquired data.
Some of these limitations have been addressed by the development of an implantable system that monitors hemodynamic status (Medtronic Chronicle, Medtronic, Inc., Minneapolis, Minn.). While this system potentially avoids the need for active patient participation, it relies on an intravascular sensor placed in the right ventricle of the heart. This approach is consistent with the prior art for implantable hemodynamic status monitoring, which has to date focused on intravascular or intramyocardial instrumentation. Examples include U.S. Pat. No. 5,454,838 in which Vallana et al. teach placement of a sensor on the myocardial wall using an intravascular approach. In U.S. Pat. No. 5,496,351, Plicchi et al. propose placing a sensor within the myocardial wall. Mortazavi in U.S. Pat. No. 5,040,538 and Cohen et al. in U.S. Pat. No. 4,815,469 describe placement of an optical sensor within the right ventricle. In the context of hemodynamic assessment for arrhythmia discrimination, Cohen and Liem (Circ., 1990, 82:394–406) study the effectiveness of a pressure transducer placed in the right ventricle. Clearly, powerful information about hemodynamic status can be obtained using intravascular instrumentation. However, intravascular or intramyocardial instrumentation carries significant risks to the patient, including increased perioperative morbidity and mortality, and increased long-term risks such as stroke and pulmonary embolism. Furthermore, intravascular instrumentation can only be performed by extensively trained specialists, thereby limiting the availability of qualified physicians capable of implanting the device, and increasing the cost of the procedure. Finally, because of the added patient risks and greater physical demands of an intravascular environment, the intravascular placement of the sensor increases the cost of development, manufacturing, clinical trials, and regulatory approval.
Though not directly related to hemodynamic status monitoring, extravascular sensing of cardiac electrical activity is known in the art. Early generations of implantable pacemakers and defibrillators relied on epicardial placement of sensing electrodes. Epicardial electrodes still see use in special patient populations. Extrathoracic sensing of cardiac electrical activity is also possible, which avoids the need for direct contact with the heart, and thus decreases the difficulty of the implant procedure and reduces the risk of perioperative complications. An example of this approach is the Reveal Insertable Loop Recorder (Medtronic, Inc., Minneapolis, Minn.), a cardiac event recorder configured for short-term implantation. As a temporarily implantable recorder, it overcomes some of the technical difficulties associated with conventional externally worn recorders of cardiac electrical activity. Two general types of externally worn recorders are Holter monitor recorders, which record continuously for an extended period of time, and cardiac event recorders, such as the King of Hearts (Alaris Medical Systems, San Diego, Calif.), which use a loop memory to retain the most recent history of cardiac electrical activity. Both these approaches require surface contact electrodes which are cumbersome and inconvenient for the patient, and more susceptible to motion artifact than an implanted electrode. However, like conventional cardiac event recorders and continuous Holter monitor recorders, the Reveal Insertable Loop Recorder is designed for short-term use as a diagnostic aid. More importantly, it requires active patient participation; when the patient recovers from a syncope, or becomes aware of symptoms, he must signal to the event recorder by means of an Activator that the recent data should be retained in long-term memory for later review by a physician. After diagnosis the Reveal Insertable Loop Recorder is explanted from the patient. Thus the Reveal is intended for short-term recording for diagnostic use, is limited to recording the electrical activity of the heart, and does not attempt to measure or quantify the hemodynamic status of the patient beyond screening for cardiac arrhythmias.
An extension of the short-term recorders just described is the Implantable Ambulatory Electrocardiogram Monitor described by Nappholz et al. in U.S. Pat. No. 5,113,869, incorporated herein by reference. This device is designed for chronic extravascular implantation. In contrast to cardiac recorders, it performs analysis on the electrocardiogram signal in order to predict imminent cardiac arrhythmias and to detect cardiac ischemia. Like the cardiac recorders, it is capable of storing raw ECG data for later review by a physician. This feature, along with the record of arrhythmic events it detected, allows the physician to tailor pharmacologic therapy. In addition, Nappholz et al. mention the use of transthoracic impedance for minute ventilation, ultrasound transducers for arterial pressure, or other sensors to allow discrimination of arrhythmias from normal cardiac rhythms caused by exertion or physiologic stress.
While the Holter monitor recorder, the Reveal Insertable Loop Recorder, and the Implantable Ambulatory Electrocardiogram Monitor provide important clinical utility in recording and monitoring cardiac electrical activity, none is designed to monitor hemodynamic status. Indeed, cardiac electrical activity does not, by itself, provide unambiguous information about hemodynamic status. By sensing only cardiac electrical activity, these devices are unable to distinguish between, for example, a hemodynamically stable cardiac rhythm and Pulseless Electrical Activity (PEA), a condition in which the heart is depolarizing normally, and thus generating a normal electrical pattern, but is not pumping blood. Furthermore, these devices are unable to recognize or quantify subtle changes in the patient's hemodynamic status. What is needed is an extravascular, hemodynamic monitor designed for chronic use.
While much of the prior art has focused on intravascular instrumentation, as discussed above, some proposal has been made to incorporate physiologic sensors into the implantable cardiac device itself Fearnot in U.S. Pat. No. 5,040,533 teaches placement of a generalized window in the housing of the cardiac device. The window might be transparent to facilitate the transmission of light or flexible to facilitate pressure transduction. While the convenience, from the clinician's perspective, of incorporating the sensors into the housing of the cardiac device is an obvious advantage, the technical difficulty in maintaining a hermetic seal between two different materials, particularly in a chronically implanted device, is equally obvious to one skilled in the art. The technical challenge is made more difficult by the greatly increased circumference, relative to that of standard feed-through connections known in the art, of the boundary between the window and the device housing. What is needed, therefore, is a method of placing a hemodynamic sensor in or on the device without compromising the integrity of the hermetic enclosure.
Prutchi et al., in U.S. Pat. No. 5,556,421 propose placement of a sensor within the header of a cardiac device. While this is an obvious solution for devices that have external leads requiring headers, it presupposes the existence of a header, and therefore does not address the implantable device that lacks a header. Furthermore, while appending a header to one end or pole of an implantable device is an efficient solution when external leads are required, appending a header-like sensor unit to one end or pole of a device not otherwise requiring a header, where the sensor unit is itself, like a header, the full-thickness of the device, is an inefficient use of volume. Thus, the approach of Prutchi et al. used in a device that doesn't otherwise require a header would be to append a header or a header-like sensor unit to one end or pole of the device, but this would unnecessarily increase both the volume and the expense of the device. A further disadvantage of placing a sensor in a header is that it does not necessarily provide for the optimal signal transduction of a particular sensor. For example, the performance of the optical sensor described in the above referenced U.S. Pat. No. 5,556,421 would be so severely degraded by direct transmission of light from source to detector that one skilled in the art would question the functionality of the proposed solution. In addition, placement in a rigid epoxy header is simply not an option for some sensors, such as sound sensors, because of the dramatic degradation in the signal-to-noise ratio the rigid header would impose. What is needed is a method of incorporating a hemodynamic sensor into a implantable device, providing it optimal access to the external milieu so that the signal of interest is optimally transduced, maintaining the hermetic enclosure provided by the device housing, and minimizing the added volume that the sensor imposes.
A solution to this challenge is offered in U.S. Pat. No. 5,404,877 by Nolan et al., in which an arrhythmia detection and warning system is described. The monitor avoids external leads and sensors by generating a radio frequency electromagnetic field within the device, which is intended to propagate through the device housing, reflect off internal organs and structures, and be detected again inside the device housing. The device uses observed changes in impedance seen at the antenna to deduce organ motion, in particular heart motion. While the leadless embodiment is desirable for the reasons described by Nolan et al., the technical challenges associated with inferring useful physiologic information from changes in impedance is obvious to one skilled in the art. The general problem is that the large number of confounding factors, e.g., changes in body position, would certainly swamp the subtle impedance changes that might result from changes in cardiac volume with contraction.
Another aspect of the prior art that has been limited is in the communication of information between a device and the clinician. During periodic follow-up in the physician's office, conventional implanted devices such as pacemakers and defibrillators are electronically interrogated and stored data is conveyed outside the body using telemetry. Depending on the physician's assessment, programmable device parameters or the patient's medical regimen may be modified. The process is initiated by the clinician and requires the placement of an external telemetry antenna in close proximity to the implanted device. Indeed, in U.S. Pat. No. 5,342,408 DeCoriolis et al. provide a telemetry signal strength indicator that facilitates the positioning of the external antenna by the clinician. While the prior art is sufficient for conventional cardiac devices, which typically only require telemetry during relatively infrequent follow-up visits, in cases where frequent telemetry is required it is desirable to have a system that does not rely on active human participation. With the Alere system described above, data is conveyed daily over telephone channels to a central location for review, a process that is initiated by the patient and requires interaction of the patient with the device. While in this case the clinician is not actively involved in the telemetry process, the patient is. This approach therefore also precludes a fully automated system. What is needed is a system that provides telemetry at a distance, so that data can be transferred remotely without the active participation of a clinician or cooperation of the patient. With telemetry at a distance, data could be automatically transferred for review and analysis by a clinician or central monitor, and programming parameters of the device could be modified remotely. By not relying on the routine participation of patient or physician, such a system would be more convenient and reliable, would avoid the inconvenience and expense of in-person follow up, and would allow frequent monitoring and tailoring of device parameters and medical therapy as the patient's disease status changes.
In U.S. Pat. No. 5,113,869 to Nappholz et al., telemetry is provided to warn the patient or physician of an impending arrhythmia event. U.S. Pat. No. 5,544,661 to Davis et al. discloses an ambulatory patient monitor that is worn by the patient and provides arrhythmia analysis and wireless two-way data and voice communication to a central station. In the event of an arrhythmia, a clinician can actively and remotely monitor the patient. However, this system is intended for short term use to monitor patients who may be subject to sudden life threatening arrhythmic events. Nappholz et al. in U.S. Pat. No. 5,720,770 discloses a system including an implanted cardiac therapy device and an external portable device in constant or periodic telemetric communication with the implanted device. The external device receives updates of the condition of the patient and the operation of the cardiac therapy device. This information is transmitted by the external device over wireless or wired phone lines to a central monitoring station.
One of the challenges of providing telemetry at a distance is to provide for the efficient transmission and reception of energy by the implanted device. The current art places the telemetry coil inside the implantable cardiac device housing and uses magnetic inductive coupling to convey data between the implanted and external units. The metallic housing attenuates the magnetic field, but since the clinician is available to actively position the external coil the degree of attenuation is tolerable. The above referenced U.S. Pat. No. 5,404,877 describes radio-frequency electromagnetic telemetry, rather than the conventional magnetic-induction method commonly used in pacemakers and implantable defibrillators. However, like the conventional magnetic coil, the RF antenna is placed within the device housing, which has the undesirable effect of attenuating the signal strength. The above referenced U.S. Pat. No. 5,113,869 discloses a radio frequency telemetry system similar to that described in above referenced U.S. Pat. No. 5,404,877, but with the antenna placed outside the device housing on a lead that extends away from the device. The configuration is desirable in that attenuation by the metallic housing of the device is avoided, however, it requires subcutaneous tunneling for placement, which causes tissue trauma and increases the risk, both acute and chronic, of infection. Furthermore, patient motion will alter the impedance between the antenna and ground plane, which degrades antenna gain. The above referenced U.S. Pat. No. 5,342,408 teaches placement of an antenna in the device header, which has the advantage of avoiding the attenuation of the metallic housing, as well as avoiding the disadvantages of an antenna that extends away from the device in a lead. However, placement in the header presupposes the existence of external leads requiring a header, which are not necessarily present in a device that uses extravascular sensors. It is desirable, therefore, to provide placement of the telemetry antenna outside the housing of a device which lacks a header. The placement should be such that the antenna is mechanically stabilized and electrically insulated from the device housing and the surrounding tissue.
Because the changes in the vascular plethysmography signal described above reflect changes in autonomic tone, monitoring the plethysmography signal provides useful information in any disease entity that influences the autonomic nervous system. In other words, the same disease entities for which HRV has proved useful are likely to also be successfully characterized using vascular plethysmography. In addition to heart failure, examples include diabetic neuropathy, detection of cardiac ischemia, arrhythmia risk stratification, sleep apnea, hypertension, and others. For a survey of disease states that may be successfully monitored using HRV, see, for example, “Heart Rate Variability: Standards of Measurement, Physiological Interpretation, and Clinical Use.” Circ. (1996), 93(5): 1043–1065.
Because of the considerations described above, it is an object of the present invention is to provide a method for use of a device that monitors a patient's hemodynamic status.
Further objects and advantages will become apparent from a consideration of the ensuing description and drawings.